The field of the present invention relates, in general, to methods and devices for manipulating and steering beams of light. More specifically, the field of the invention relates to a compact, hybrid system of optical components that accept a beam of light from an optical element or fiber, steer the beam in one or more directions, and pass the deflected beam into open space or through a secondary optical element or fiber.
Investigators of optical phenomenon typically rely on massive, vibration damped optical benches to maintain precise alignment between optical elements during prototyping. Optical elements might consist of lenses, mirrors, beam splitters, piezoelectric actuators, translation tables, prisms, screens, lasers, optical fibers, gratings, etc. Quite commonly, these elements are macroscopic in size and can easily be handled and adjusted. Although suitable for most optical prototyping purposes, the use of macroscopic optical elements can have its drawbacks.
For example, to precisely steer a beam of light at a high angular rate, one might employ conventional piezo motor, or angular galvanometer, and mirror assembly. Using two such devices at right angles to one another in the same optical path would give two degrees of freedom for controlling the path of the beam. This arrangement is commonly used for steering laser beams in "laser show" productions. In this application, the physiological demands of human eyesight require only a 30 to 60 Hertz refresh cycle of each laser scanned frame to provide the illusion of smooth motion. Given that the reflected laser light is of adequate intensity at the maximum angular slew rate, the overall angular size and detail of a single frame will be limited by the total path length traced out to form that frame. That is to say, the angular extent of a laser image is limited by the maximum angular slew rate of each steering mirror.
One apparent solution might be to increase the torquing capability of the mirror driving motor. This is effective to a point. With increasing torque capability, the angular inertial mass of the rotor elements becomes ever larger. At some point, the mechanical dynamics of the coupled motor mirror system will suffer. Unwanted torsional deflections will be introduced that result in beam steering errors. Stiffening the rotor might remedy the deflection problem, but would again increase the angular inertial mass. Thus, motor sizing is not a complete panacea.
A better solution would be to significantly reduce the size, and therefore, the mass and angular inertia of the moving mirror. There is no torque penalty in doing so. However, care must be taken to insure that the stiffness of the lighter mirror remains high so that dynamic distortion of the mirror itself does not compromise optical performance.
Beam steering devices and their functional equivalents, are found in a wide variety of products including laser bar scanners. CD ROM heads, laser printers, optical switches, robotic vision scanners, optical choppers, optical modulators and display devices to name a few.
In the field of micromechanics, a number of recent developments in the area of spatial light modulators (SLM), light valves, and deformable mirror devices (DMD) have resulted in a significant cost reduction and a substantial increase in performance of beam steering devices.
TEXAS INSTRUMENTS holds a number of DMD patents including U.S. Pat. Nos. 5,504,614 and 5,448,546 that describe methods of fabricating electrically controllable micromirrors using an additive process. The micromirrors may operate independently or within a distributed array. The micromirrors are generally of torsion or gimbaled hinge design.
An additional patent U.S. Pat. No. 5,325,116 held by TEXAS INSTRUMENTS describes a beam steering device used for writing to and reading from an optical storage media using a micromachined SLM. Although the single SLM component has the potential to greatly improve the mechanical dynamic response of the overall device, the surrounding structure within which the SLM resides remains bulky.
What is needed is a faster, more precise and compact apparatus for steering beams of light. In particular, it would be advantageous to miniaturize a complete optical system upon a single substrate, including lenses, optical fibers, optical sensors, SLMs and the like. Not only would performance be greatly enhanced due to smaller moving masses, but manufacturing costs and uniformity would also be improved.
Reducing all dimensions proportionately on a given mirror design results in a reduction of surface area by an inverse squared term and a reduction of volume and mass by an inverse cubed term. Thus, by diminishing the size of any object, the surface-area-to-mass-ratio will increase linearly. Consequently, surface force reactions such as surface tension, electrostatics and Van der Waals forces, become more significant, while gravitational and inertial forces becomes less of a factor in governing the static and dynamic equations of motion.
The angular inertia of a rectangular plate about a center line lying in the plane of the plate, is linearly proportional to the mass of the plate. It is also proportional to the square of the width of the plate which is perpendicular to that axis. Therefore, if all dimensions of a plate are reduced by a factor of two, then the final mass would be the inverse cube of two, or one eighth of the original mass. The inertial mass, sometimes referred to as the mass moment of inertia, of the smaller plate could then be one eighth times the inverse square of two or one thirty-second times the original mass.
Since the angular acceleration of a body is directly proportional to an externally applied torque and inversely proportional to its angular inertial mass, one can conclude that halving all plate dimensions will result in thirty two fold increase in angular acceleration for a given torque.
As is commonly known, electrostatic force is an effective means for moving small micromachined components. The force produced between an electrostatically charged plate and around is directly proportional to the plate's surface area and inversely proportional to the square of the plate-to-ground gap for a given voltage. Thus, if all dimensions are again halved, the electrostatic force generated between the plate and ground would be equal to the initial force for a given voltage.
By taking the previous dynamic and electrostatic arguments into consideration, it can be surmised that by halving all dimensions of an electrostatically driven plate, the dynamic response would be improved by a factor of thirty two for a given driving voltage. More generally, assuming that the driving voltage is such that electrostatic breakdown of air and insulators does not occur, then the dynamic response of an electrostatically driven plate will increases as the inverse forth power of size reduction.